This invention relates generally to a driver circuit for presenting digital data to a transmission line and more specifically to an improved circuit wherein transformer-type coupling may be used to transmit both symmetrical and nonsymmetrical codes.
In known prior art transmission line driver circuits in which the pulse code modulated signal is coupled to a transmission line by way of a transformer, problems arise when an attempt is made to send nonsymmetrical codes due to the storage of energy in the magnetic field of the coupling transformer. For example, when the pulse train being transmitted is unbalanced with respect to a DC baseline, i.e., the voltage-time product of the positive pulses is different from that of the negative pulses, the field in the coupling transformer stores energy, resulting in a shift of the DC baseline. This shift may be sufficiently large so as to cause errors in the data being received at the remote end of the transmission line.
Certain codes, such as non-return to zero (NRZ) and Miller codes are inherently nonsymmetrical. These codes offer an advantage over symmetrical codes, such as phase modulated or frequency modulated codes, in that more information or a higher bit density may be packed into them than can be accomplished with symmetrical codes. Accordingly, it is desirable to be able to transmit such nonsymmetrical code patterns over long transmission lines through the use of transformer coupling between the coded data to be transmitted and the lines themselves.
The present invention provides a transmission line driver circuit which receives as an input, the digital data to be transmitted and which employs transformer coupling to the transmission line. More specifically, first and second transformers are provided which have their secondary windings connected in an opposite sense to the terminals of the transmission line by means of suitably poled diodes. The primary winding of the first transformer is connected between a source of reference potential and a waveform shaping circuit by means of a first transistor. Similarly, the primary winding of the second transformer is coupled between the source of reference potential and a second wave shaping circuit by means of a second transistor. Suitable logic circuits are provided for causing the first and second transistors to be operated in a complementary fashion. That is, when the transistor associated with the primary winding of the first transformer is conducting, the transistor associated with the primary winding of the second transformer is nonconducting and vice versa. In operation, when one transformer is providing the signal (negative or positive), the other transformer has its stored field reduced to zero so that when the transformer is driven to apply a digital pulse to the transmission line it operates from a condition whereby no energy is initially stored in it.